Ecofys, P.O. Box 8408, 3503 RK Utrecht, The Netherlands, tel. +3130-2808300, fax +3130-2808301, A.Schaap@Ecofys.nl

Gramsbergen, Tus 7, 5507 MG Veldhoven, The Netherlands, tel. +3140-2301500, fax +3140-2301499,

Gramsbergen.solar@iae.nl

The solar heating system with heat pump and ice storage is a novel concept for the application of solar heat in energy efficient houses (see figure). The heat demand in a Dutch energy efficient single family house for space heating amounts to about 12 GJ/year and for domestic hot water about 8 GJ/year (floor area typically about 100 m²).

The solar thermal collectors are being used as a heat source for the (electric) heat pump (about 3 kW). However when the ambient temperatures are low (below 0 ^oC) and the solar irradiance is low, then the contribution of the solar collectors (about 10 m²) is insufficient. During these periods the heat is withdrawn from an "ice storage" with a content of about 3 m³. The uninsulated ice storage (maximum temperature about 15 ^oC) can be buried under or next to the house. The water in this storage can be transformed into ice. In this way also the very high latent heat of the water is used to enlarge the thermal capacity. A temporary storage of about 300 liter for the sewage (toilet excluded) is placed in the ice storage. In this way the ice storage recovers heat from the sewage in wintertime (about 5 GJ/winter). Between solar collectors and heat pump a daily storage is placed. This storage acts like a domestic hot water system in summer. During the heating season the daily storage improves the COP (Coefficient of Performance of the heat pump) by storing the somewhat higher temperatures coming from the solar collectors. Without the daily storage the heat pump is always connected to the ice storage. It would operate on a source temperature around 0 ^oC all winter.

In the summer the combination of daily storage, hot water storage and solar collectors delivers directly hot water to the house. In spring and fall the solar collectors can also bypass the heat pump for the deliverance of (low temperature) space heating to the house, if a high enough temperature is available. For this bypass a heat exchanger is installed parallel to the heat pump (not in the figure).

Such a system can be an alternative for the withdrawl of heat from the surrounding (ambient air, ambient water or ground). Financed by NOVEM (Netherland agency for energy and the environment) the feasabillity of the system was determined. A simulation program was developed with which not only this system but also the competing heat sources could be simulated. Competing heat sources for a system for a single family house are the vertical or horizontal ground heat exchanger or the ambient air heat exchanger. Because of the high temperatures from the solar collectors in spring, fall and especially in summer; the SPF (seasonal performance factor) can be much higher than by utilisation of ambient heat only. Values of the SPF upto five can be reached.

In most cases uncovered collectors are being used as a heat source for heat pumps. From the simulations it was concluded that in combination with an ice storage with sewage heat recovery, spectrally selective covered collectors gave a higher SPF. The costs per avoided ton of emission of carbondioxid are also lower. It appeared that covered collectors have two advantages in this combination:

• In summer covered collectors can deliver domestic hot water directly to the house.
• In winter the ice storage with heat recovery restrains the heat source temperature to around 0 ^oC. With outside temperatures below 0 ^oC the contribution of uncovered collectors is lower than that of covered collectors.

The costs of such a heat source (solar collectors, daily storage and ice storage with heat recovery) are about US$ 5100. Competing heat sources cost about US$ 1900. Because of the higher SPF the costs per avoided emission of a ton of carbondioxid is about equal to that of the competing heat sources (around US$ 750 per ton per year). The reference system is a high efficiency natural gas fired heating and hot water system. The calculations are based on the Dutch electricity and natural gas costs for small consumers; respectively US$ 0.12 per kWh and US$ 0.29 per m³ of natural gas.

With financial support from NOVEM the system is built by Gramsbergen in a new housing scheme in Veldhoven in the Netherlands. The system will be monitored extencively. Especially the heat transfer to and from the ice storage will be examined in detail, because the ice storage with heat recovery is the most innovative part of the system. The monitoring results after the first winter 98/99 will be presented at the conference.

Keywords: Solar Heating, heat pumps, ice stotrage, heat recovery